Methods of Fabricating Semiconductor Devices with Enlarged Recessed Gate Electrodes

ABSTRACT

A semiconductor device includes a semiconductor substrate having a recess therein. A gate insulator is disposed on the substrate in the recess. The device further includes a gate electrode including a first portion on the gate insulator in the recess and a second reduced-width portion extending from the first portion. A source/drain region is disposed in the substrate adjacent the recess. The recess may have a curved shape, e.g., may have hemispherical or ellipsoid shape. The source/drain region may include a lighter-doped portion adjoining the recess. Relate fabrication methods are also discussed.

REFERENCE TO PRIORITY APPLICATION

This application is a divisional of U.S. application Ser. No. 10/738,316, filed Dec. 17, 2003, which claims priority to Korean Application No. 2002-81091, filed Dec. 18, 2002. The disclosure of U.S. application Ser. No. 10/738,316 is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to semiconductor devices and methods of fabrication therefor, and more particularly, semiconductor devices with recessed gate electrode portions and methods for forming the same.

An integrated circuit (IC) may include several metal oxide semiconductor (MOS) transistors. As ICs become more highly integrated, the scale of such MOS transistors may decrease. Therefore, there is a need for reducing problems arising from short channel effects for such small-scale MOS transistors.

A gate of a MOS transistor may be formed by a method wherein a gate insulator is formed on a semiconductor substrate and then a gate conductive layer is formed on the gate insulator and patterned. Source/drain regions may be formed by performing ion-implantation process after forming the gate electrode.

However, the gate formed by a conventional patterning process may have etch damage on sidewalls thereof, so that charge may inconstantly flow from the channel to the periphery of the gate and leakage of current may occur. Additionally, conventional methods may not be capable of forming a gate electrode having a shorter channel length than a resolution of a photolithography process. This may serve as a limit on operation speed and power consumption characteristics.

In order to solve problems due to etch damage and to improve characteristics of a semiconductor device, a T-shaped gate electrode formed by a damascene process has been introduced as shown, for example, in U.S. Pat. No. 6,255,202.

FIGS. 1 through 4 are cross-sectional views showing a conventional method for forming a T-shaped gate electrode. Referring to FIG. 1, an oxide layer 13 and a nitride layer 15 are sequentially formed on a semiconductor substrate 11. The nitride layer 15 is patterned using a photolithography process, thereby forming an initial groove 17 exposing the oxide layer 13. Referring to FIG. 2, spacers 19 are formed on sidewalls of the initial groove 17. The exposed oxide layer 13 is patterned using the spacers 19 as etch masks, thereby exposing the semiconductor substrate 11.

Referring to FIG. 3, the spacers 19 are removed to form a final groove 21. A gate oxide layer 23 is formed on the exposed semiconductor substrate 11, and the final groove 21 is filled with a conductive material, thereby forming a T-shaped gate electrode 25. Although not illustrated, the nitride layer 15 is removed and an ion-implantation process is performed to form source/drain regions (not shown) in a subsequent process.

In order to reduce problems arising from a short channel effect, source/drain regions are typically formed very shallowly. However, as semiconductor devices become increasingly highly integrated, it is generally more difficult to form source/drain regions very shallowly in a process of forming a T-shaped gate or a conventional gate. To address these problems, a process of forming a recessed gate electrode portion (or an elevated source/drain) has been introduced, as described, for example, in U.S. Pat. No. 6,303,448, which discloses a process for forming a recessed gate electrode portion (or an elevated source/drain).

FIGS. 5 through 7 are cross-sectional views showing a conventional method of forming a recessed gate electrode portion. Referring to FIG. 5, an oxide layer 53 and a nitride layer 55 are sequentially formed on a semiconductor substrate 51. The nitride layer 55, the oxide layer 53 and a part of the semiconductor substrate 51 are anisotropically dry-etched to form a groove 57. Referring to FIG. 6, a gate insulator 63 is formed on the surface of the etched semiconductor substrate 51 by a thermal oxidation process. Referring to FIG. 7, the groove 57 is filled with a conductive material, thereby forming a recessed gate electrode portion 65. Although not illustrated, the nitride layer 55 is removed and an ion-implantation process is performed to form source/drain regions in a subsequent process.

The conventional methods described above may have several problems. According to a conventional process of forming a T-shaped gate electrode, it may be difficult to form source/drain regions with shallow junctions. This may be because the channel typically is formed around a surface of a semiconductor substrate like a conventional gate process. Also, drain-induced barrier lowering DIBL may occur, which may cause a leakage current between the semiconductor substrate and a gate insulator at a lower voltage than a threshold voltage.

According to some conventional processes of forming a recessed gate electrode portion, a capacitance of a parasitic capacitor is increased due to overlap of gate electrode and source/drain. This is because the semiconductor substrate is partially etched to enlarge a contact area between the gate electrode and the semiconductor substrate and also a gate insulator of a thermal oxide is thinly conformally formed along an inner wall of a groove formed on the semiconductor substrate. Gate-induced drain leakage (GIDL) may be increased in a device fabricated in such a manner.

SUMMARY OF THE INVENTION

According to some embodiments of the present invention, a semiconductor device includes a semiconductor substrate having a recess therein. A gate insulator is disposed on the substrate in the recess. The device further includes a gate electrode including a first portion on the gate insulator in the recess and a second reduced-width portion extending from the first portion. A source/drain region is disposed in the substrate adjacent the recess. The recess may have a curved shape, e.g., may have a hemispherical or semielliptical shape. The source/drain region may include a lighter-doped portion adjoining the recess.

In some embodiments, the gate insulator comprises a first portion disposed on a sidewall of the recess and having a first thickness, and a second portion disposed on a bottom of the recess and having a second thickness less than the first thickness. The first portion of the gate insulator may adjoin a source/drain region in the substrate.

In further embodiments of the present invention, the device further includes an insulation layer on a surface of the substrate adjoining the second portion of the gate electrode above the substrate and extending over a portion of the first portion of the gate electrode. An insulating spacer is disposed on a sidewall of the second portion of the gate electrode and on the insulation layer.

In further embodiments of the present invention, the gate electrode further comprises a third portion on the second portion, the third portion having a greater width than the second portion. An insulation layer on a surface of the substrate may adjoin the second portion of the gate electrode above the substrate and extend over a portion of the first portion of the gate electrode, and an insulating spacer may be disposed on a sidewall of the second portion of the gate electrode, on a sidewall of the third portion of the gate electrode and on the insulation layer. In some embodiments, the gate insulator may include a first portion disposed on a sidewall of the recess and having a first thickness, and a second portion disposed on a bottom of the recess and having a second thickness less than the first thickness. In other embodiments, the gate insulator comprises a substantially uniform thickness insulation layer lining the recess.

In some method embodiments of the present invention, a semiconductor device is formed. An insulation layer is formed on a substrate, and a sacrificial layer is formed on the insulation layer. An opening through the sacrificial layer and the insulation layer is formed to expose an active region of the substrate. A portion of the active region is removed to form a recess (e.g., having a hemispherical, semielliptical or other curved shape) therein that has a greater width than the opening through the sacrificial layer and the insulation layer. A gate insulator is formed in the recess and a gate electrode is formed extending through the sacrificial layer and the insulation layer and into the recess. The gate electrode includes a first portion on the gate insulator in the recess and a second reduced-width portion extending from the first portion. The sacrificial layer is removed to expose a sidewall of the gate electrode outside of the recess, and an insulating spacer is formed on the exposed sidewall of the gate electrode. A source/drain region is formed in the substrate adjacent the recess.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 4 are cross-sectional views showing a conventional method of forming a T-shaped gate electrode.

FIGS. 5 through 7 are cross-sectional views showing a conventional method of forming a recessed gate electrode portion.

FIG. 8 is a cross-sectional view showing a semiconductor device according to some embodiments of the present invention.

FIG. 9 is a cross-sectional view showing a semiconductor device according to further embodiments of the present invention.

FIG. 10 is a cross-sectional view showing a semiconductor device according to still further embodiments of the present invention.

FIGS. 11 through 25 are cross-sectional views showing operations for forming the semiconductor device of FIG. 8.

FIGS. 26 through 31 are cross-sectional views showing operations for forming the semiconductor device of FIG. 9.

FIGS. 32 through 36 are cross-sectional views showing operations for forming the semiconductor device of FIG. 10.

FIGS. 37 through 40 are cross-sectional views showing operations for forming a semiconductor device according to additional embodiments of the present invention.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. It will be understood that when an element such as a layer, region or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. Furthermore, relative terms, such as “beneath,” may be used herein to describe one element's relationship to another element as illustrated in the Figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. For example, if a device in the figures is turned over, elements described as “below” other elements would then be oriented “above” the other elements. The exemplary term “below,” therefore, encompasses both an orientation of above and below.

It will be understood that although the terms first and second are used herein to describe various regions, layers and/or sections, these regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one region, layer or section from another region, layer or section. Thus, a first region, layer or section discussed below could be termed a second region, layer or section, and similarly, a second without departing from the teachings of the present invention. Like numbers refer to like elements throughout.

FIG. 8 is a cross-sectional view showing a semiconductor device 800 according to some embodiments of the present invention. Referring to FIG. 8, an active region electrically isolated by a field oxide 113 is defined on a semiconductor substrate 111. The semiconductor substrate 111 includes a recess 125 r. A surface of the recess 125 r has a smoothly curved shape. For example, the recess 125 r may have a semielliptical or hemispherical shape.

A gate electrode 135 fills the recess 125 r and has a predetermined height. An insulator 132 is interposed between the semiconductor substrate 111 and a portion of the gate electrode 135 filling the recess 125 r, i.e., a recessed gate electrode portion portion 135 r on a surface of the recess 125 r. The recess 125 r may have a thickness of at least about 300 Å, for example.

The gate electrode 135 may include the recessed gate electrode portion portion 135 r filling the recess 125 r and a protruding gate electrode portion 135 p protruding from the semiconductor substrate 111 and connected to the recessed gate electrode portion portion 135 r. The protruding gate electrode portion 135 p may include a bottom gate electrode portion portion 135 b and a main gate electrode portion portion 135 m. The bottom gate electrode portion portion 135 b has a shorter length than a width of the recess 125 r. The main gate electrode portion portion 135 m is connected to the bottom gate electrode portion portion 135 b. The main gate electrode portion portion 135 m has a longer length than a width of the recess 125 r. The main gate electrode portion portion 135 m may include a main gate electrode center portion portion 135 c on the bottom gate electrode portion portion 135 b and a main gate electrode portion silicide spacer 137 on sidewalls of the main gate electrode center portion portion 135 c. An end of the recess 125 r is located between an outer end of the silicide spacer 137 and an outer end of the bottom gate electrode portion portion 135 b. The gate electrode thus has a shape of an anchor. The recessed gate electrode portion portion 135 r, the bottom gate electrode portion portion 135 b and the main gate electrode center portion portion 135 c may be formed of polysilicon.

In this specification, the gate length indicates a size of a gate electrode along a direction of a channel. The silicide spacer 137 is for decreasing resistance of the gate electrode and may be formed by a CVD method.

The insulator 132 interposed between the recessed gate electrode portion portion 135 r and the semiconductor substrate 111 may include a buffer gate insulator 129 a and a gate insulator 131 of different thicknesses. The buffer gate insulator 129 a is located on sidewalls of the recess 125 r. The gate insulator 131 is thinner than the buffer gate insulator 129 a and connected to the buffer gate insulator 129 a on a bottom of the recess 125 r. Preferably, the gate insulator 131 is arranged on a bottom of the bottom gate electrode portion portion 135 b, that is, the buffer gate insulator 129 a preferably is located on both sidewalls of the recess 125 r of both sidewalls of the bottom gate electrode portion portion 135 b. The thick buffer gate insulator 129 a serves to decrease a parasitic capacitance between the gate electrode 135 and the semiconductor substrate 111 (i.e., source/drain). The thin gate insulator 131 serves as an effective gate insulator. Thus, a short channel transistor may be provided without increase of a parasitic capacitance by properly controlling thicknesses of the buffer gate insulator 129 a and the gate insulator 131.

A buffer insulator 115 a surrounds the bottom gate electrode portion portion 135 b of the gate electrode 135 and is aligned with the outer end of the silicide spacer 137. Also, a gate insulation spacer 141 is disposed on outer sidewalls of the silicide spacer 137 and the buffer insulator 115 a. The buffer insulator 115 a and the gate insulation spacer 141 isolate the gate electrode 135.

A low concentration impurity-doped region 139 is disposed in the semiconductor substrate 111 under the buffer insulator 115 a and the gate insulation spacer 141. A high concentration impurity-doped region 143 is disposed in the semiconductor substrate 111 out of the gate insulation spacer 141 and connected to the low concentration impurity-doped region 139. The impurity-doped regions 139 and 143 may be source/drain regions. As described above, a parasitic capacitance occurring due to an overlap of the gate electrode and the source/drain may be decreased due to the thick buffer gate insulator 129 a. Silicide layers 147 may be disposed on the gate electrode 135 and on the semiconductor substrate 111 out of the gate insulation spacer 141, i.e., on the high concentration impurity-doped region.

FIG. 9 is a cross-sectional view showing a semiconductor device 900 according to further embodiments of the present invention. The device 900 is similar to the semiconductor device 800 of FIG. 8, with like elements indicated by like reference numerals. The device 900 of FIG. 9 differs from the device 800 of FIG. 8 in that a gate electrode 135′ of the device 900 does not include a silicide spacer.

Referring to FIG. 9, a gate electrode 135′ includes a recessed gate electrode portion portion 135 r′ filling a recess 125 r of a semiconductor substrate 111 and a protruding gate electrode portion 135 p′ connected to the recessed gate electrode portion portion 135 r′. The protruding gate electrode portion portion 135 p′ includes a bottom gate electrode portion portion 135 b′ and a main gate electrode portion portion 135 m′. The bottom gate electrode portion 135 b′ portion connects to the recessed gate electrode portion portion 135 r′ and has a gate length that is less than a width of the recess 125 r. The main gate electrode portion portion 135 m′ connects to the bottom gate electrode portion portion 135 b′ and has a gate length greater than a width of the recess 125 r.

Similar to the semiconductor device 800 of FIG. 8, an insulator 132 is interposed between the recessed gate electrode portion 135 r′ and the semiconductor substrate 111. The insulator 132 is composed of a buffer gate insulator 129 a and a gate insulator 131 of different thicknesses. The buffer gate insulator 129 a is thicker than the gate insulator 131 and located on sidewalls of the recess 125 r. The gate insulator 131 is located on a bottom of the recess 125 r. The protruding gate electrode portion 135 p′ of the gate electrode 135′ is T-shaped. The protruding gate electrode portion 135 p′ and the recessed gate electrode portion portion 135 r′ have an anchor shape.

A nitride liner 130 a may be disposed on sidewalls of the bottom gate electrode portion portion 135 b′ and on the buffer gate insulator 129 a. The device 800 further includes a gate insulation spacer 141 and a buffer insulator 115 a. The gate insulation spacer 141 is disposed on sidewalls and a bottom surface of the main gate electrode portion portion 135 m′. The buffer insulator 115 a surrounds the bottom gate electrode portion portion 135 b′. The buffer insulator 115 a is aligned with the gate insulation spacer 141. The gate insulation spacer 141 is L-shaped.

Similar to the semiconductor device of FIG. 8, a low concentration impurity-doped region 139 is disposed in the semiconductor substrate 111 under the gate insulation spacer 141 and the buffer insulator 115 a. A high concentration impurity-doped region 143 is disposed in the semiconductor substrate 111 under the gate insulation spacer 141. The gate electrode 135′ may be formed of polysilicon.

FIG. 10 is a cross-sectional view showing a semiconductor device 1000 according to still further embodiments of the present invention. The device 1000 is similar to the device 900 of FIG. 9, with like items indicated by like reference numerals. The device 1000 differs from the device 900 of FIG. 9 in that a recess of the device 1000 is shallower than a corresponding recess of the device 900, and an insulator formed on a surface of the recess has a single insulation layer with a constant thickness.

Referring to FIG. 10, the semiconductor device 1000 includes a semiconductor substrate 111 having a recess 125 r′. The recess 125 r′ has a depth of about 50 Å. A gate electrode 135″ includes a recessed gate electrode portion portion 135 r″ filling the recess 125 r′ of the semiconductor substrate 111 and a protruding gate electrode portion 135 p″ connected to the recessed gate electrode portion portion 135 r″. The protruding gate electrode portion 135 p″ includes a bottom gate electrode portion portion 135 b″ and a main gate electrode portion portion 135 m″. The bottom gate electrode portion portion 135 b″ connects to the recessed gate electrode portion portion 135 r″ and has a gate length that is less than a width of the recess 125 r′. The main gate electrode portion portion 135 m″ connects to the bottom gate electrode portion portion 135 b″ and has a gate length that is less than the width of the recess 125 r″. An insulator 132′ is interposed between the recessed gate electrode portion portion 135 r″ and the semiconductor substrate 111. The insulator 132′ is a single insulation layer with a constant thickness.

FIGS. 11 through 25 are cross-sectional views showing exemplary operations for forming the semiconductor device 800 of FIG. 8. Referring to FIG. 11, a field oxide 113 is formed on a semiconductor substrate 111 using, for example, a conventional process. An active region is defined by the field oxide 113. The conventional process may be, for example, a shallow trench isolation (STI) process.

A pad oxide layer 115, a sacrificial nitride layer 117 and an optional planarization buffer layer 119 are sequentially formed on the semiconductor substrate 111. The optional planarization buffer layer 119 is formed of a material having an etch selectivity with respect to the sacrificial nitride layer 117, for example, an oxide. The pad oxide layer 115 may be formed by, for example, a thermal oxidation process or a CVD process. The sacrificial nitride layer 117 and the selective planarization buffer layer 119 may be formed by using, for example, a CVD process. A thickness of the sacrificial nitride layer 117 is substantially the same as a height of a gate electrode that will be formed in a subsequent process. The optional planarization buffer layer 119 will be patterned in a subsequent patterning process, so that the sacrificial nitride layer 117 has a desirable profile (i.e., a gate electrode having a vertical sidewall may be formed). The optional planarization buffer layer 119 may not be formed according to a process condition.

A photo resist pattern 121 is formed on the optional planarization buffer layer 119. The photoresist pattern 121 may be formed by using the following steps: The semiconductor substrate 111 is coated with a photoresist layer, which is exposed to light by using a predetermined reticle to form the photoresist pattern 121. The photoresist pattern 121 has an opening 123 therein. The opening 123 defines a shape of a gate electrode that will be formed in a subsequent process.

A gate electrode groove may be formed by using either of two methods as illustrated in FIGS. 12A, 13A and 14A and in FIGS. 12B, 13B and 14B. Referring to FIG. 12A, the optional planarization buffer layer 119 and the sacrificial nitride layer 117 exposed by the opening 123 in the photoresist pattern 121 are anisotropically etched until the pad oxide layer 115 is exposed, thereby forming a main gate electrode groove 125 m in the sacrificial nitride layer 117. Referring to FIG. 13A, the photoresist pattern 121 is removed, and a groove spacer 127 is formed on inner sidewalls of the main gate electrode groove 125 m, thereby forming a main gate electrode center portion groove 125 mc having a shorter width than the main gate electrode groove 125 m. Thus, it is possible to form a channel having a short width exceeding resolution capability of a photolithography process. The groove spacer 127 may be formed of silicon nitride or high-density plasma enhanced tetraethylorthosilicate (TEOS). In order to form the groove spacer 127, a nitride layer or a high-density plasma oxide layer is conformally formed on the semiconductor substrate 111 where the photoresist pattern 121 is removed, and the nitride or plasma oxide layer is then anisotropically etched. Referring to FIG. 14A, the exposed pad oxide layer 115 is etched using the groove spacer 127 as an etch mask, thereby forming a bottom gate groove 125 b defined by the pad oxide 115 that exposes a portion of the semiconductor substrate 111.

An alternative way of forming a protruding gate electrode groove is explained by referring to FIGS. 12B, 13B and 14B. Referring to FIG. 12B, the optional planarization buffer layer 119 and a part of the sacrificial nitride layer 117 are anisotropically etched using the photoresist pattern 121 as an etch mask, thereby forming a main gate electrode groove 125 m′ that exposes a remaining sacrificial nitride layer 117. The sacrificial nitride layer 117 is totally etched to expose the pad oxide pattern 115 in FIG. 12A.

Referring to FIG. 13B, a groove spacer 127′ is formed on sidewalls of the main gate electrode groove 125 m′ by using the technique as described above with reference to FIG. 13A, thereby defining a main gate electrode center portion groove 125 m′c′. Referring to FIG. 14B, the remaining sacrificial nitride layer 117 and the pad oxide layer 115 are anisotropically etched using the groove spacer 127′ as an etch mask, which exposes a portion of the semiconductor substrate 111. A bottom gate electrode groove 125 b′ is formed through the remaining sacrificial nitride layer 117 and the pad oxide layer 115. The technique of FIGS. 12B, 13B and 14B has a difference of height of the bottom gate electrode portion groove in comparison with the technique of FIGS. 12A, 13A and 14A.

Subsequent processes to those shown in FIGS. 12A, 13A and 14A will now be described. Referring to FIG. 15, after forming the bottom gate electrode groove 125 b, the exposed semiconductor substrate 111 is isotropically etched to form a recess 125 r. The recess 125 r may be formed by a wet etching or chemical dry etching. The recess 125 r has a width that is narrower than a width of the main gate electrode groove 125 m of FIG. 12A and wider than a width of the bottom gate electrode groove 125 b. The contour of the recess 125 r may be a smooth curve, such as a hemispherical or semielliptical shape. The recess 125 r may have a thickness greater than about 300 Å.

Referring to FIG. 16, a buffer gate insulator 129 is formed at a surface of the recess 125 r and has first thickness. Fort example, the buffer gate insulator 129 may be formed by a thermal oxidizing process and with a thicker thickness than a desired thickness of a gate insulator. A channel ion-implantation process is performed to prevent punch through and control threshold voltage. In particular, impurity ions are implanted in the semiconductor substrate 111 under the recess 125 r through the buffer gate insulator 129.

Referring to FIG. 17, the buffer gate insulator 129 is anisotropically etched. Namely, anisotropic dry etch is carried out on the gate insulator 129 to remove a portion of the gate insulator 129 on the bottom portion 125 rb of the recess 125 r exposed by the bottom gate electrode grove 125 b. Also, wet etch can be employed to remove a portion of the gate insulator 129 on the bottom portion 125 rb of the recess 125 r exposed by the bottom gate electrode grove 125 b. As a result, a buffer gate insulator 129 a remains on sidewalls of the recess 125 r, and a bottom portion 125 rb of the recess 125 r having a width about the same as the width of the bottom gate electrode groove 125 b is exposed.

Referring to FIG. 18, a gate insulator 131 is formed on the bottom portion 125 rb of the recess 125 r. The gate insulator 131 has a thickness less than the thickness of the buffer gate insulator 129 a. The gate insulator 131 may be formed by thermal oxidation.

Although not illustrated, after the buffer gate insulator 129 is formed, a nitride liner may be formed. The nitride liner may be anisotropically etched to expose the buffer gate insulator 129. Then, the exposed buffer gate insulator 129 may be isotropically etched to expose a portion of the semiconductor substrate 111. A thermal oxidation process may then be performed to form a thinner gate insulator than the buffer gate insulator on the exposed semiconductor substrate 111.

Referring again to FIG. 18, after forming the gate insulator 131 on the bottom 125 rb of the recess 125 r, a gate electrode material layer 133 is formed on the optional planarization buffer layer 119 to fill the recess 125 r, the bottom gate electrode groove 125 b through the pad insulator 115, and the main gate electrode center portion groove 125 mc through the sacrificial nitride layer 117. The gate electrode material layer 133 may comprise, for example, polysilicon.

Referring to FIG. 19, a planarization process is performed until the sacrificial nitride layer 117 is exposed, thereby forming a recessed gate electrode portion 135 r in the recess 125 r, a bottom gate electrode portion 135 p in the bottom gate electrode groove 125 b and a main gate electrode center portion 135 c in the main gate electrode center portion groove 125 mc. The planarization process may be, for example, a chemical mechanical polishing (CMP) or an etch back process. The sacrificial nitride layer 117 functions as a planarization stopping layer during the planarization process.

Referring to FIG. 20, the sacrificial nitride layer 117 and the groove spacer 127 are removed to expose the main gate electrode center portion 135 c. If the groove spacer 127 is formed of a nitride, the sacrificial nitride layer 117 and the groove spacer 127 may be removed using the same solution, such as phosphoric acid. If the groove spacer 127 is formed of high-density plasma enhanced TEOS, the sacrificial nitride layer 117 may be removed first by using the phosphoric acid, and then the groove spacer 127 may be removed by using a conventional oxide etch solution.

Referring to FIG. 21, a silicide spacer 137 is formed at sidewalls of the exposed main gate electrode center portion 135 c. The gate electrode center portion 135 c and the silicide spacer 137 form a main gate electrode portion 135 m, and the main gate electrode portion 135 m and the bottom gate electrode portion 135 b form a protruding gate electrode portion 135 p. The protruding gate electrode portion 135 p and the recess gate electrode portion 135 r form a gate electrode 135 having a shape of an anchor.

In order to form the silicide spacer 137, a metal silicide layer may be formed by a conventional method, such as CVD, and etched back. A metal silicide layer may be conformally formed on the gate electrode center 135 c and the pad oxide layer 115 and the metal silicide layer is etched back until the pad oxide layer 115 is exposed. As a result, a metal silicide layer remains on sidewalls of the main gate electrode center portion portion 135 c as a spacer. The silicide spacer 137 may be formed of, for example, tungsten silicide or cobalt silicide. A barrier layer may be formed before forming the silicide spacer 137. The barrier layer may be formed of, for example, titanium silicide.

Referring to FIG. 22, after forming the silicide spacer 137, the exposed pad oxide layer 115 is etched back such that the portion of pad oxide layer 115 that does not underlie the silicide spacer 137 is removed to expose the semiconductor substrate 111 and a portion 115 a of the pad oxide layer 115 a remains under the silicide spacer 137. A low concentration impurity-doped region 139 may then be formed into the exposed semiconductor substrate 111 adjacent to the silicide spacer 137 by implanting impurity ions of low concentration to form a lightly doped drain (LDD) region. The low concentration impurity-doped region 139 preferably has a depth that is substantially the same as that of the recess 125 r. Depths of the low concentration impurity-doped region and a subsequent high concentration impurity-doped region may be dependent on the depth of the recess 125 r.

Referring to FIG. 23, a gate insulation spacer 141 is formed on sidewalls of the remaining pad oxide 115 a and the silicide spacer 137. The gate insulation spacer 141 may be formed from a nitride material. In order to form the gate insulation spacer 141, a nitride layer may be conformally formed and etched back.

Referring to FIG. 24, impurity ions of high concentration are implanted into the semiconductor substrate 111 adjacent to the gate insulation spacer 141 using the gate insulation spacer 141 as an ion-implantation mask, thereby forming a high concentration impurity-doped regions 143. The high concentration impurity-doped regions 143 and the low concentration impurity-doped regions 139 serve as source/drain regions. The high concentration impurity-doped region 143 may be more deeply formed than the low concentration impurity-doped region 139.

The ion-implantation process for formation of the low concentration impurity-doped region may be omitted. That is, if the recess 125 r is formed to a depth of about 800 Å, the ion-implantation process for formation of the low concentration impurity-doped region may be omitted. In such a case, when the ion-implantation process for formation of the high concentration impurity-doped region is performed, the low concentration impurity-doped region may be simultaneously formed according to a concentration profile. However, the ion-implantation process for formation of the low concentration impurity-doped region may be omitted even though the recess has a different depth.

Referring to FIG. 25, a metal layer 145 comprising cobalt, titanium or nickel is formed on the semiconductor substrate 111 on the exposed semiconductor substrate 111, on the gate insulation spacer 141 and on an upper surface of the gate electrode 135. The metal layer 145 is thermally treated to form a silicide layer 147 on the high concentration impurity-doped region 143 and on an upper surface of the gate electrode 135.

Operations for forming the semiconductor device 900 of FIG. 9 will now be explained with reference to FIGS. 26 through 31. Discussion of like operations described above with reference to FIGS. 10 through 25 will be omitted. Referring to FIG. 26, a gate electrode center groove 125 m′c′ and a bottom gate electrode groove 125 b are formed as described above with reference to FIGS. 11, 12B, 13B and 14B. A groove spacer 127′ is removed to form a main gate electrode groove 125 m and a bottom gate electrode groove 125 b. The exposed semiconductor substrate 111 is isotropically etched to form a recess 125 r. A width of the recess 125 r is less than that of the main gate pattern groove 125 m but greater than that of the bottom gate electrode groove 125 b.

Referring to FIG. 27, a buffer gate insulator 129 of a first thickness is formed on a surface of the recess 125 r using a thermal oxidation process. A nitride liner 130 is formed on the semiconductor substrate 111 where the buffer gate insulator 129 is formed, i.e., on the buffer gate insulator 129, on inner walls of the gate electrode groove 125 and on the planarization buffer layer 119. The nitride liner 130 may be formed using a CVD process.

Referring to FIG. 28, an etch back process is performed to remove the nitride liner 130 on the bottom 125 rb of the recess 125 r and expose a part of the buffer gate insulator 129. The exposed part of the buffer gate insulator 129 is etched to expose the bottom 125 rb of the recess 125 r of the semiconductor substrate 111. The buffer gate insulator on both sidewalls 125 rs of the recess 125 r is not removed. The removal of the exposed part of the buffer gate insulator 129 can be done by dry etch or wet etch. A gate insulator 131 is formed on the bottom 125 rb of the recess 125 r, and is relatively thinner than the buffer gate insulator 129. The gate insulator 131 may be formed by using a thermal oxidation process.

Referring to FIG. 29, a conductive material, such as polysilicon, is formed to fill the gate electrode groove 125 and planarized down to the sacrificial nitride layer 117, thereby forming a gate electrode 135′. The gate electrode 135′ includes a recessed gate electrode portion 135 r′ filling the recess 125 r, a bottom gate electrode portion 135 b′ filling the bottom gate electrode groove 125 b and a main gate electrode portion 135 m′ filling the main gate electrode portion groove 125 m.

Referring to FIG. 30, the sacrificial nitride layer 117 is removed using an etch solution, such as phosphoric acid. The nitride liner on the sidewalls of the main gate electrode portion 135 m′ and the remaining sacrificial nitride layer 117 may be simultaneously removed. Thus, a bottom surface of the main gate electrode portion 135 m′ may be partially exposed. Using operations described above, a low concentration impurity-doped region 139 may be formed by implanting impurity ions of low concentration using the gate electrode 135′ as an ion-implantation mask.

Referring to FIG. 31, a nitride layer is formed on the resultant structure having the low concentration impurity-doped region 139. An etch back process is performed with respect to the nitride layer to form a gate insulation spacer 141. The etch back process is performed until the buffer oxide layer 115 is exposed. An etch back process is then performed with respect to the exposed buffer oxide layer 115, thereby exposing the semiconductor substrate 111 that does not underlie the gate insulation spacer 141 while leaving a buffer oxide layer 115 a under the gate insulation spacer 141. A high concentration impurity-doped region 143 is formed by implanting impurity ions of high concentration using the gate insulation spacer 141 as an ion-implantation mask.

Operations for forming the semiconductor device 1000 of FIG. 10 will now be explained with reference to FIGS. 32 through 36. Discussion of like operations described above will be omitted. Referring to FIG. 32, described above with reference to FIGS. 11, 12A, 13A and 14A, after forming a main gate electrode center portion groove 125 mc and a bottom gate electrode groove 125 b, a groove spacer 127 is removed, thereby forming a main gate electrode groove 125 m and a bottom gate electrode groove 125 b. The exposed semiconductor substrate 111 is isotropically etched to form a recess 125 r′. The recess 125 r′ may have a very shallow depth, e.g., about 50 Å. The recess part 125 r′ has a width not exceeding that of the main gate electrode groove 125 m.

Referring to FIG. 33, a gate insulator 132′ is formed on a surface of the recess 125 r′. The gate insulator 132′ may be formed by using a thermal oxidation process. The gate insulator 132′ has a constant thickness, in contrast to gate insulators of the aforedescribed embodiments. A gate electrode material 133 is formed on the planarization buffer layer 119, thereby filling the gate electrode groove 125.

Referring to FIG. 34, a planarization process is performed until the sacrificial nitride layer 117 is exposed, thereby forming a gate electrode 135″. The gate electrode 135″ includes a recessed gate electrode portion 135 r″ filling the recess 125 r′, a bottom gate electrode portion 135 b″ filling the bottom gate electrode groove 125 b, and a main gate electrode portion 135 m″ filling the main gate electrode groove 125 m.

Referring to FIG. 35, the sacrificial nitride layer 117 is removed, thereby exposing the pad oxide layer 115 and the main gate electrode portion portion 135 m″ of the gate electrode 135″. A low concentration impurity-doped region 139 is formed using an ion-implantation process.

Referring to FIG. 36, a nitride layer is formed on the resultant structure where the low concentration impurity-doped region 139 is formed. The nitride layer is etched back to form a gate insulation spacer 141. An etch back process is performed until the buffer oxide layer 115 is exposed. The exposed buffer oxide layer 115 is etched back to expose the portion of the semiconductor substrate 111 that does not underlie the gate insulation spacer 141, while the buffer oxide 115 remains under the gate insulation spacer 141. A high concentration impurity-doped region 143 is formed by implanting impurity ions at high concentration using the gate insulation spacer 141 as an ion-implantation mask.

The recess part 125 r′ is shallowly formed. The recess 125 in the embodiments described above with reference to FIGS. 11 through 25 may be likewise shallowly formed, i.e., with a shallow depth of about 50 Å. This will be briefly explained by referring to FIGS. 37 through 40.

Referring to FIG. 37, a main gate electrode center portion groove 125 mc and a bottom gate electrode groove 125 b are formed by using, for example, the operations described above with reference to FIGS. 11, 12A, 13A, 14A and 15. An exposed semiconductor substrate 111 is isotropically etched to form a shallow recess 125 r′. The recess 125 r′ may have a depth of about 50 Å. A thermal oxidation process is then performed to form a gate insulator 132 on the shallow recess 125 r′. A gate electrode material 133 is formed on the selective planarization buffer layer 119, thereby filling the recess 125 r′, the bottom gate electrode groove 125 b through the pad oxide layer 115 and the main gate electrode center portion groove 12 mc through the sacrificial nitride layer 117.

Referring to FIG. 38, a planarization process is performed until the sacrificial nitride layer 117 is exposed, thereby forming a recessed gate electrode portion 135 r′″ filling the recess 125 r′, a bottom gate electrode portion 135 b′″ filling the bottom gate electrode groove 125 b and a main gate electrode center portion portion 135 c′″ filling the main gate electrode center portion 125 mc.

Referring to FIG. 39, the sacrificial nitride layer 117 and the groove spacer 127 are removed to expose the main gate electrode center portion 135 c. A silicide spacer 137 is formed on both sidewalls of the exposed main gate electrode center portion 135 c. Thus, a gate electrode 135′″ is formed. In particular, the main gate electrode center portion 135 c′″ and the silicide spacer 137 form the main gate electrode portion 135 m′″, and the bottom gate electrode portion 135 b′″ and the main gate electrode portion ′″ 135 m form a protruding gate electrode portion 135 p′″. The protruding gate electrode portion 135 p′″ and the recessed gate electrode portion 135 r′″ form a gate electrode 135′″ having a shape of an anchor.

After forming the silicide spacer 137, the exposed pad oxide layer 115 is etched back, thereby removing the portion of the pad oxide layer 115 that does not underlie the silicide spacer 137 and exposing the semiconductor substrate 111. A portion 115 a of the pad oxide layer remains under the silicide spacer 137. A low concentration impurity-doped region 139 is formed in the exposed semiconductor substrate 111 adjacent to the silicide spacer 137 by implanting impurity ions of low concentration, thus forming an LDD region.

Referring to FIG. 40, a gate insulation spacer 141 is formed on sidewalls of the remaining pad oxide layer 115 a and the silicide spacer 137. A high concentration impurity-doped region 143 is formed in the semiconductor substrate 111 adjacent to the gate insulation spacer 141 by implanting impurity ions at a high concentration. The high concentration impurity-doped region 143 and the low concentration doped region 139 serve as source/drain regions.

According to some embodiments of the present invention, a recess is formed in a semiconductor substrate. A thin gate insulator is formed on a bottom of the recess and a thick gate insulator is formed on sidewalls thereof. Thus, parasitic capacitance between the semiconductor substrate and a gate electrode in the recess may be reduced part due to the thick gate insulator on the sidewalls of the recess. Because an effective gate insulator is formed on the bottom of the recess, a semiconductor device having a short channel and exhibiting high speed and/or low power consumption can be provided. Additionally, a silicide layer may be formed on sidewalls of a gate electrode, thereby decreasing a resistance of a semiconductor device.

In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims. 

1. A method of forming a semiconductor device, comprising: forming an insulation layer on a substrate; forming a sacrificial layer on the insulation layer; forming an opening through the sacrificial layer and the insulation layer to expose an active region of the substrate; removing a portion of the active region to form a recess therein that has a greater width than the opening through the sacrificial layer and the insulation layer; forming a gate insulator in the recess; forming a gate electrode extending through the sacrificial layer and the insulation layer and into the recess, the gate electrode comprising a first portion on the gate insulator in the recess and a second reduced-width portion extending from the first portion; removing the sacrificial layer to expose a sidewall of the gate electrode outside of the recess; forming an insulating spacer on the exposed sidewall of the gate electrode; and forming a source/drain region in the substrate adjacent the recess.
 2. The method as claimed in claim 1, wherein the forming a gate insulator comprises: forming a first insulating layer having a first thickness in the recess; removing a portion of the first insulating layer on a bottom of the recess to expose a bottom of the recess while leaving a portion of the first insulating layer on a sidewall of the recess; and forming a second insulation layer on the exposed bottom of the recess, the second insulation layer having a second thickness less than the first thickness.
 3. The method of claim 2, wherein removing a portion of the first insulating layer is preceded by forming a nitride liner on the first insulation layer, wherein removing a portion of the first insulating layer on a bottom of the recess to expose a bottom of the recess while leaving a portion of the first insulating layer on a sidewall of the recess comprises removing portions of the first insulating layer and the nitride liner on the bottom of the recess to expose the bottom of the recess while leaving portions of the first insulating layer and the nitride liner on a sidewall of the recess.
 4. The method of claim 1, wherein removing a portion of the active region to form a recess therein is preceded by forming a sidewall spacer on a sidewall of the sacrificial layer and the insulation layer, and wherein removing a portion of the exposed active region comprises etching the exposed active region using the sidewall spacer as an etching mask.
 5. The method of claim 4, wherein forming a gate electrode comprises depositing a conductive material through an opening defined by the sidewall spacer.
 6. The method of claim 1, wherein forming an opening comprises: removing a portion of the sacrificial layer overlying the active region to expose the insulation layer; forming a sidewall spacer on a sidewall of the sacrificial layer; and etching an exposed portion of the insulation layer using the sidewall spacer as an etching mask to expose a portion of the active region.
 7. The method of claim 1, wherein forming an opening comprises: removing a portion of the sacrificial layer overlying the active region without exposing the insulation layer; forming a sidewall spacer on a sidewall of the sacrificial layer; and etching through portions of the sacrificial layer and the insulation layer using the sidewall spacer as an etching mask to expose a portion of the active region.
 8. The method of claim 7, wherein forming a gate electrode comprises: removing the sidewall spacer to form an enlarged opening through at least one portion of the sacrificial layer; and depositing conductive material into the enlarged opening.
 9. The method of claim 1: wherein forming an insulating spacer comprises forming a first insulating spacer on the exposed sidewall of the gate electrode; wherein forming a source/drain region comprises implanting ions into the substrate using the first insulating spacer as an implantation mask; wherein forming an insulating spacer further comprises forming a second insulating spacer on the first insulating spacer; and wherein forming a source/drain region comprises implanting ions into the substrate using the second insulating spacer as an implantation mask to thereby form a source/drain region comprising lighter and heavier doped portions.
 10. The method of claim 1, further comprising forming a planarization buffer layer on the sacrificial layer, wherein forming an opening through the sacrificial layer and the insulation layer to expose an active region of the substrate comprises forming an opening through the planarization buffer layer, the sacrificial layer and the insulation layer, and wherein forming a gate electrode extending through the sacrificial layer and the insulation layer and into the recess comprises: depositing a conductive material in the opening through the planarization buffer layer, the sacrificial layer and the insulation layer; and planarizing the substrate to remove the planarization buffer layer and a portion of the deposited conductive material overlying the sacrificial layer.
 11. A semiconductor device comprising: a semiconductor substrate having a recess; a gate electrode comprising a recessed gate electrode portion filling the recess and a protruding gate electrode portion connected to the recessed gate electrode portion and protruding from the semiconductor substrate; and an insulator interposed between the semiconductor substrate and the recessed gate electrode portion, wherein the protruding gate electrode portion comprises a bottom gate electrode portion connecting to the recessed gate electrode portion and having a narrower width than that of the recess and a main gate electrode portion connecting to the bottom gate electrode portion and having a wider width than that of the recess.
 12. The semiconductor device as claimed in claim 11, wherein the insulator comprises: a buffer gate insulator having a first thickness on both sidewalls of the recess; and a gate insulator having a second thickness thinner than the first thickness and connecting to the buffer gate insulator on a bottom of the recess.
 13. The semiconductor device as claimed in claim 12, wherein ends of the gate insulator are aligned with outer sidewalls of the bottom gate electrode portion.
 14. The semiconductor device as claimed in claim 13, wherein the main gate electrode portion comprises a main gate electrode center portion having the same gate length with the bottom gate electrode portion and connecting to the bottom gate electrode portion; and a silicide spacer disposed on both sidewalls of the main gate electrode center portion, and an end point of the recess is located between an outer sidewall of the silicide spacer and an outer sidewall of the bottom gate electrode portion.
 15. The semiconductor device as claimed in claim 12, further comprising a nitride liner disposed on a part of the buffer gate insulator and on both sidewalls of the protruding gate electrode portion.
 16. The semiconductor device as claimed in claim 13, further comprising: a buffer insulator formed on both sidewalls of the bottom gate electrode portion and aligned with outer sidewalls of the metal silicide spacer; an insulation spacer disposed on outer sidewalls of the buffer insulator and the metal silicide spacer; and a metal silicide layer disposed on an upper surface of the main gate electrode center portion and on the semiconductor substrate adjacent to both sidewalls of the insulation spacer.
 17. The semiconductor device as claimed in claim 11, further comprising: an insulation spacer disposed on outer sidewalls and bottom surfaces of the main gate electrode portion; and a buffer insulator disposed on outer sidewalls of the bottom gate electrode portion and aligned with outer sidewalls of the insulation spacer.
 18. The semiconductor device as claimed in claim 16, wherein the buffer insulator is formed of oxide and the insulation spacer is formed of nitride.
 19. The semiconductor device as claimed in claim 17, wherein the buffer insulator is formed of oxide and the insulation spacer is formed of nitride.
 20. The semiconductor device as claimed in claim 11, wherein the recess has a smooth curved surface.
 21. The semiconductor device as claimed in claim 17, further comprising: a low concentration impurity-doped region in the semiconductor substrate under the buffer insulator and the insulation spacer; and a high concentration impurity-doped region connecting to the low concentration impurity-doped region in the semiconductor substrate adjacent to the insulation spacer.
 22. The semiconductor device as claimed in claim 16, further comprising: a low concentration impurity-doped region in the semiconductor substrate under the buffer insulator and the insulation spacer; and a high concentration impurity-doped region connecting to the low concentration impurity-doped region in the semiconductor substrate adjacent to the insulation spacer. 